Charging xerographic images

ABSTRACT

APPARATUS FOR CHARGING SURFACES, PARTICULARLY XEROGRAPHIC SURFACES, USING A RELATIVE MOTION BETWEEN A POINT AND A SURFACE TO PROVIDE A RADIAL SWEEP OF A SLOPE SUCH THAT RELATIVELY UNIFORM CHARGE ACCEPTANCE TAKES PLACE OVER EACH INCREMENTAL AREA.   D R A W I N G

Jan. 12, 1971 I sMlTH ETAL 3,555,378

CHARGING XEROGRAPHIC IMAGES Filed Bay 14, 1968 2 Sheets-Sheet I Jan. 12,1971 s y ETAL 3,555,378 r CHARGING XEROGRAPHIC IMAGES Filed Hay, 1968 2Sheets-Sheet 2 United States Patent Oflice 3,555,378 Patented Jan. 12,1971 3,555,378 CHARGING XEROGRAPHIC IMAGES Ian E. Smith and Kenneth A.Metcalfe, Lockleys, South Australia, Australia, assignors to TheCommonwealth of Australia, The Secretary Department of Supply, Parkes,Australian Capital Territory, Australia Filed May 14, 1968, Ser. No.729,094 Claims priority, application Australia, May 15, 1967, 21,718/ 67Int. Cl. H01t 19/04 US. Cl. 317262 7 Claims ABSTRACT OF THE DISCLOSURE\Apparatus for charging surfaces, particularly xerographic surfaces,using a relative motion between a point and a surface to provide aradial sweep of a slope such that relatively uniform charge acceptancetakes place over each incremental area.

This invention relates to apparatus for charging surfaces, such asxerographic surfaces or insulating membranes, and in particular itrelates to the charging of surfaces by means of a corona.

It is customary in charging surfaces to provide a series of points, or-a wire or the like, to which a sufficiently high voltage is applied toproduce a corona discharge, and to pass this over the surface which isto be charged.

It has been shown that it is important to use the correct part of such adischarge for the actual charging of the surface, because differentareas of such a discharge have a different charging effect, and also,when using a series of points as is customary there is a lack ofuniformity in the field produced within each of the corona fields, andalso between each of the corona fields generated by each point. Thuseach corona pattern spreads outwards from the point at which it isgenerated, with a maximum field density effect at the center of thedischarge and a gradually decreasing intensity outwards therefrom, andfor this reason, When using a series of corona discharge points, thereare a number of independent patterns generated, one for each point,which do not overlap due to repulsion effects, and therefore it isnecessary to move the medium being charged in relation to the points totry to cause averaging of the charge at all areas and to avoid absenceof charging at fringe areas.

Whichever way this movement is effected, it is found impossible toobtain a completely uniform charge acceptance on the entire area of themedium which is subjected to such charging.

For this purpose, instead of using a series of points, it is thepractice to use a wire or other laterally extending member so that thereis a concentrated effect along a plane, but if the section of the wireor member varies, or if there is variation of its distance fromany partof the surface being charged, there will be a difference in chargeintensity and effect, and although there is relative movement of thewire and surface at right angles to the plane of the wire, chargingbands are produced along which the intensity can vary. Although, it hasbeen proposed heretofore to so arrange the corona that the selected partor band of the field produced by such a discharge is actually effectiveon the surface, the above problems make uniform charging very difficultif not impossible, and the primary object of this invention therefore isto allow a more uniform charge to be obtained than was heretoforepossible, and secondly to achieve this in a simpler manner.

The objects of the invetion are achieved by moving a. single coronapoint relative to the work piece being charged in such a manner that therequired band selectively sweeps the surface about a radius to produce arelatively uniform charge acceptance.

The use of a single point removes the problem of multiplenon-overlapping fields or varying fields over a length, but this thenintroduces a further problem, namely, that the charge levels vary from amaximum central zone to a minimum fringe zone, and thus if relativelyuniform charge areas are required, only small areas are available, andthis effect will be demonstrated later herein with reference to theillustrations, but according to this invention this problem is overcomeby using a single basic corona discharge point so arranged that itsfield extends sufficient- 1y to cover at least slightly more than onehalf of the area to be charged, and this area or the point is moved theone in relation to the other, with the result that relatively uniformcharge acceptance is obtained over a large area.

It is possible to move both the workpiece and the discharge member, orto move only the discharge member in relation to a workpiece in such amanner that this uniform charging results, but it will be realized alsothat by suitable positioning, a corona needle or the like outside of thenormal area which is being charged, the unwanted part of the dischargezone can be kept outside of the actual charge area of the requiredsurface.

To enable the invention to be fully understood, an embodiment will nowbe described with reference to the accompanying drawings which areillustrative only and are not to be taken as limiting the invention, thescope of which is defined in the claims herein.

In the drawings:

'FIG. 1 is a side elevation view of the apparatus,

FIG. 2 is a plan view of same, indicating the corona field density,

FIG. 3 is a graph showing the charge density effect at different coronadistances from a workpiece, and

FIG. 4 shows a modified apparatus.

Referring first to FIGS. 1 to 3, the sheet 1 to be charged is placed ona rotating disc 2 with a charging point 3 positioned outside theperiphery of the disc 2, and a high voltage is applied between thecharging point 3 and the disc 2. It will then be appreciated readily,that in the central zone 4 of the corona 5 emanating from the point 3,the charging rate is highest and the incremental area of the disc 2traverse is greatest, whereas in the outer corona zone 6 the chargingrate is least where the incremental area traversed is least. A uniformcharge is generated on the sheet using such an arrangement.

T o understand this reference should be made to FIG. 3 where the X axisshows distance or extension of the corona while the Y axis shows thefield intensity at the surface member being charged along the extensionwhen measured at various heights above the work piece, from which itwill be seen that at a height of one inch there is a high fieldintensity at the Y axis but a short extension along the X axis, whereasat a height of six inches the field intensity is relatively lower at theY axis but extends considerably along the X axis, the actual distanceusiug 15 kv. direct current being about eight or nine 3 inches. Byselecting an appropriate part of such a zone, say the part designated R,and revolving the work piece 1 through this about the axis A, arelatively uniform charge acceptance is obtainable over an area of some12 or more inch circle.

It will of course 'be appreciated that because of incremental areainvolved the denser field at the Y axis is effective at each area onlyfor a short time during each revolution of the disc 2 whereas at theaxis A the charge is continuously maintained. Thus, as said, byappropriately selecting the slope as indicated in FIG. 3 to be in theratio of the radius, which can be done by a correct selection of theheight of the point to the spread required and varying the voltage orcorona shape accordingly.

It will be realized that this overcomes the problem which has existedheretofore where a single point to plane corona produced a circularcharge pattern on an insulating sheet, with the maximum density at thecenter decreasing radially outward in the method used heretofore, andthe maximum charge density increasing with charge interval. When thecharge density at the center reaches a critical value, a change occurswhich is characterized by a marked reduction in electroradiographicspeed and quality. The diameter of this central zone increases withtime. Optimum image quality and X-ray response occurs at the outer edgeof the central zone, and image density decreases radially outward.

The present invention however, produces a uniform charge density over alarge area, this charge being limited to a maximum just below thecritical value which causes the central zone type of charging. On theother hand with the disc charging device referred to above, when thedisc or baseplate rotates, the rate of charge acceptance is reduced atthe center of the corona and increased in the outer zone of the corona.The corona envelope, of charge density decreasing from the patterncenter outwards, is matched to the moving electroradiographic member insuch a manner that the charge acceptance increases from the coronacenter outwards. Charge uniformity and charging time for a particularsize of electrophotographic member is adjusted experimentally by varyingthe height and the distance of the corona point from the center of thedisc for a given applied voltage as stated.

It will be realized that the invention can be variously applied but ineach case there will be a relative motion between the corona producingdevice and the surface on which the charge is being developed and thiswill be so arranged that there is a scanning of the whole of the area bya part of the corona discharge which will produce a relatively uniformcharge over the required area.

It is possible to use a shield as 7, see FIG. 1, which can be held at avoltage to modify the pattern or projecting of the corona by introducinga repulsion or attraction effect.

In the drawings the high voltage generator 13 and the driving motor 14for the disc 2 are housed in the base 8, voltage controls 9 beingprovided, the point 3 being adjustable up and down on the stem 10, bymeans of collar 11 with locking knob 12 of conventional construction andif required the point 3 may be axially adjustable in collar 11 to allowa required pattern to be achieved. Generally it is advisable to avoidthe center of the corona discharge and to select the starting point somedistance in from the Y axis as shown by the left limit of R in FIG. 3 asthis tends to have the wrong slope and characteristic for chargingpurposes, and of course the right limit of R must fall Within thecharging density line.

While generally a single point has been referred to, it is possiblewithin the spirit of the invention to use more than one point providedthe repulsion effect is maintained so that it does not prevent theextension of at least one of the fields over the axis of rotation, thisbeing usable where perhaps an edge portion requires to be filled in witha heavier charge.

Also instead of charging a single work piece extending across the axisof rotation, a number of work pieces could be placed around the outerpart of the disc for successive charging, in which case the field neednot necessarily extend over the axis of rotation.

Such an embodiment is shown in FIG. 4 where 15 represents a discrotating about the axis 16 and carrying a series of work pieces 17 whichpass through the corona zones 18 generated from the points 19. In thisembodiment the work pieces 17 can be placed into position at S on thedisc 15 and are then carried through the charge zones 18 to be finallyremoved at F.

From the foregoing it will be appreciated that the corona, or a seriesof coronas, are directed to charge a xerographic surface which hasrelative rotation to the point about an axis perpendicular to thesurface, and when an area of charge is required which is longer than theradius of the corona at the xerographic surface the point, or one of thepoints only, is arranged to have its field extended from an area outsideof the xerographic surface inwards to beyond the axis of rotation, withthe center of the corona. preferably just outside of the edge of thearea being charged.

We claim:

1. Apparatus for charging surfaces comprising: a disc on which a memberhaving an insulating layer can be carried, at least one corona dischargepoint disposed above said disc, said disc being rotatable about an axisperpendicular to the plane of the disc, means to rotate the disc, andmeans to apply a voltage between the discharge point and the said disc,said point being so disposed in relation to the said rotational axisthat the center of the corona is disposed near the outer edge of thearea being charged with part of the corona extending inward toward theaxis of rotation, whereby a relatively uniform charging per incrementalarea is attained during rotation.

2. Apparatus according to claim 1 wherein a single point is used whichis disposed approximately over the edge of the said disc and facesgenerally toward the said disc.

3. Apparatus according to claim 1 wherein the discharge point isvariable at least in its distance above the said support, and the meansto apply the voltage between the discharge point and the disc isvariable whereby the pattern of the corona may be selected.

4. Apparatus according to claim 1 comprising a shield disposedrearwardly of the said point to assist control of the corona pattern.

5. Apparatus for charging surfaces comprising: a support on which amember having an insulating layer can be carried, said support beingplanar and having an edge, a single corona discharge point disposedabove the said support approximately at the edge thereof and facing saidsupport, said discharge point and support being arranged to allowrelative rotational motion about an axis perpendicular to the plane ofthe support, means to cause the relative motion, and means to apply avoltage between the discharge point and the said support, said pointbeing so disposed in relation to the said rotational axis that thecenter of the corona is disposed near the outer edge of the area beingcharged with part of the corona extending inward toward the axis ofrotation, whereby a relatively uniform charging per incremental area isattained during rotation.

6. Apparatus for charging surfaces comprising: a sup port on which amember having an insulating layer can be carried, at least one coronadischarge point disposed above the said support arranged to allowrelative rotational motion about an axis perpendicular to the plane ofthe support, means to cause the relative motion, and means to apply avoltage between the discharge point and the said support, said pointbeing so disposed in relation to the said rotational axis that thecenter of the corona is disposed near the outer edge of the area beingcharged with part of the corona extending inward toward the axis ofrotation and said discharge point being so positioned in relation to theedge of the area being charged that the center part of the corona isoutside of the charging area whereby a relatively uniform charging perincremental area is attained during rotation.

7. Apparatus according to claim '6 wherein the discharge point is sopositioned in relation tothe edge of the area being charged that thecenter corona extends inwards to just beyond the rotational axis.

References Cited UNITED STATES PATENTS 3,241,466 3/1966 Clark 355-31,250,140 12/1917 Chapman 317262 3 3,146,687 9/1964 Donelson et al.355-3 2,043,217 6/1936 Yaglou 317-4 J D MILLER, Primary Examiner C. L.YATES, Assistant Examiner US. 01. X.R.

